The present invention relates to a process for preparing alkoxylated carbonyl compounds of the general formula I (compounds I)
R1aR2C (OR3)bxe2x80x83xe2x80x83I
where
R1 and R2 are each hydrogen or C1-C6-alkyl,
R3 is independently at each instance C1-C6-alkyl,
a is 0 or 1 and
b is 2 or 3,
with the proviso that the sum total of a and b is 3, by anodic oxidation of geminal alkoxy compounds of the general formula II (compounds II) 
where
R4, R5, R6 and R7 are each hydrogen or C1-C6-alkyl and
R5 and R6 are each C1-C6-alkyl or C1-C6-alkoxy,
in the presence of a C1-C6-alkyl alcohol (compounds III), which comprises using a cathodic depolarizer comprising a customary organic compound (compounds IV) that is suitable for electrochemical reduction and conducting the anodic oxidation and the cathodic reduction in an undivided electrolytic cell in the presence of C1-C6-alkyl alcohols.
The preparation of organic compounds by concurrently utilizing the cathode reaction and the anode reaction has already been the focus of intensive research work on account of its particularly high energy efficiency (see M. M. Baizer, Organic Electrochemistry, 3rd Ed. (Eds. H. Lund and M. M. Baizer), Marcel Dekker, Chapter 35, New York 1991).
Although there are scientific papers (cf. Nonaka and Li, Electrochemistry, 67, 1999 Jan., 4-10, 1999) pointing out that there is in principle a multitude of coproduction possibilities, concrete industrial teaching is to be found in the scientific literature only for a few, and usually specific, examples.
Apart from a few mixtures (cf. DE-A-19618854) it has been determined that coproduction electrosynthesis is associated with technical disadvantages which rule out large scale industrial use in practice. These include in particular the difficult separation of the resulting reaction mixtures and also chemical reactions of reactants and products at the respective counterelectrodes, whereby the yield of the desired products of value is much reduced when the reaction is carried out in undivided electrolytic cells. The use of divided electrolytic cells would avoid these disadvantages, it is true, but these cell designs are very capital intensive. Especially in organic electrolytes, commercially available ion exchange membranes possess only very limited stability that rules out sustained industrial use.
J. Amer. Chem. Soc., (1975) 2546 and J. Org. Chem., 61 (1996) 3256 and Electrochim. Acta 42, (1997) 1933 disclose electrochemical processes whereby a Cxe2x80x94C single bond between carbon atoms which each carry an alkoxy function can be oxidatively cleaved.
DE-A-10043789, unpublished at the priority date of the present invention, describes the production of orthoesters from alkoxylated diketones.
However, neither of the last two references cited suggests that these production processes might be useful in the realm of coproduction electrosynthesis.
It is an object of the present invention to provide a coproduction electrosynthesis process that combines the preparation of alkoxylated carbonyl compounds by anodic oxidation with the preparation of high value added organic compounds in a cathodic reduction and that does not have the aforementioned disadvantages of customary coproduction syntheses and, more particularly, provides the desired products of value in high yields.
We have found that this object is achieved by the process described above.
It is particularly favorable to use 1,2-di(C1-C6-alkoxy)ethane or 1,2-di(C1-C6-alkoxy)propane or 1,1,2,2-tetra(C1-C6-alkoxy)ethane or 1,1,2,2-tetra(C1-C6-alkoxy)propane (compounds II). The compounds I produced in the process are the corresponding formaldehyde di(C1-C6-alkyl) acetals or tri(C1-C6-alkyl) orthoformates and in the case of the propane derivatives as starting materials likewise acetaldehyde di(C1-C6-alkyl) acetals or tri(C1-C6-alkyl) orthoacetates. The aforementioned acetaldehyde and acetic acid derivatives are likewise preparable from 2,3-di(C1-C6-alkoxy)butane.
This is a particularly simple way of obtaining especially formaldehyde dimethyl acetal, trimethyl orthoformate, acetaldehyde dimethyl acetal and trimethyl orthoacetate from the corresponding compounds II and methanol.
As well as the aforementioned di- or tetraalkoxy ethane or -propane derivatives, useful compounds I and II include especially those where R4 has the same meaning as R7 and R5 the same meaning as R6 in order that the number of compounds in the reaction mixture to be worked up may be minimized.
Generally, alcohols will be used whose alkyl radicals have the same meanings as R8 and R9 or as the alkyl radicals in R5 and R6, provided R5 and R6 are each C1-C6-alkoxy.
Useful cathodic depolarizers are customary organic compounds that are suitable for anodic reduction, such as aromatic hydrocarbyl compounds, activated olefins, carbonyl compounds, aromatic carboxylic acids and derivatives thereof and also naphthalene or ring-substituted naphthalene derivatives.
The process of the invention is particularly useful for preparing the following compounds or classes of compounds:
a) maleic acid or maleic acid derivatives where the acid function is in the form of alkyl esters into tetraalkyl butanetetracarboxylates by hydrodimerization,
b) benzenemono-, -di- or -tricarboxylic acids other than phthalic acid or phthalic acid derivatives, or benzenemono-, -di- or -tricarboxylic acid derivatives where the acid function is in the form of alkyl esters or derivatives substituted on the aromatic nucleus, into the corresponding mono-, di- and triformylbenzene compounds where the formyl groups are present in the form of an acetal,
c) acrylic acid, alkyl acrylates, acrylamide or acrylonitrile or homologues thereof into the corresponding hydrodimerization products; preferred homologues are those of the general formula V
R10xe2x80x94CHxe2x95x90CHxe2x80x94Xxe2x80x83xe2x80x83V
xe2x80x83where X is an alkoxycarbonyl, nitrile or carbamide group and R10 is C1-C6-alkyl,
d) phthalic acid, alkyl phthalates or derivatives thereof substituted on the aromatic nucleus, into phthalide or ring-substituted phthalide derivatives, cyclohexane- or cyclohexene-1,2-dicarboxylic acid, dialkyl cyclohexane- or cyclohexene-1,2-dicarboxylates or derivatives substituted on the cyclohexane or cyclohexene ring in correspondence with the substitution pattern of the phthalic acid derivatives that are substituted on the aromatic nucleus,
e) naphthalene or ring-substituted naphthalene derivatives into 1,2,3,4-tetrahydronaphthalene or the corresponding 1,2,3,4-tetrahydronaphthalene derivatives,
f) pyridine or ring-substituted pyridine derivatives into 1,4-dihydropyridine or the corresponding 1,4-dihydropyridine derivatives.
Alkyl ester groups in reactants or products are in particular C1-C6-alkyl ester groups.
Useful substituents for substitution on the aromatic rings in the aforementioned starting compounds include inert, difficult-to-reduce groups such as C1-C12-alkyl, C1-C6-alkoxy or halogen.
As regards the phthalide or phthalide derivatives mentioned under point d), these are in particular compounds as described in DE-A-19618854.
Said reference likewise provides a more particular description of particularly suitable starting compounds.
The molar ratio of the starting compounds for cathode and anode reactions and also of the thereby formed products in the electrolytes relative to each other is uncritical.
Generally the molar ratio of the sum total of compounds I and II to the alcohols (compounds IV) will be in the range from 0.1:1 to 5:1, preferably in the range from 0.2:1 to 2:1, particularly preferably in the range from 0.3:1 to 1:1.
Conducting salts included in the electrolysis solution will generally be alkali metal, tetra(C1-C6alkyl)ammonium or tri(C1-C6-alkyl)benzylammonium salts. Useful counterions include sulfate, hydrogen sulfate, alkyl sulfates, alkyl sulfonates, halides, phosphates, carbonates, alkyl phosphates, alkyl carbonates, nitrate, alkoxides, tetrafluoroborate or perchlorate.
Useful conducting salts further include the acids derived from the aforementioned anions.
Preference is given to methyltributylammonium methosulfate (MTBS), methyltriethylammonium methosulfate or methyltripropylmethylammonium methosulfates.
The electrolysis solution may include customary cosolvents. These are inert solvents having a high oxidation potential which are generally customary in organic chemistry. Examples are dimethyl carbonate and propylene carbonate.
The process of the invention may be carried out in any customary undivided electrolytic cell type. It is preferable to operate a continuous process using undivided flowthrough cells. Stack plate cells having stack electrodes connected in series as described for example in DE-A-19533773 are particularly suitable.
The current densities used in the process are generally in the range from 1 to 1000 mA/cm2, preferably in the range from 10 to 100 mA/cm2. The temperatures are generally in the range from xe2x88x9220 to 60xc2x0 C., preferably in the range from 0 to 60xc2x0 C. The process is generally carried out at atmospheric pressure. Higher pressures are preferably reserved for the use of higher temperatures, in order that boiling of the starting compounds or cosolvents may be avoided.
Useful anode materials include for example noble metals such as platinum or metal oxides such as ruthenium or chromium oxide or mixed oxides of the RuoxTiox type. Preference is given to graphite or coal electrodes.
Useful cathode materials include for example iron, steel, stainless steel, nickel or noble metals such as platinum and also graphite or coal materials. Preference is given to a system utilizing graphite as anode and cathode and also graphite as anode and nickel, stainless steel or ordinary steel as cathode.
After the reaction is ended, the electrolyte solution is worked up by general methods of separation. For this, the electrolysis solution is generally first distilled and the individual compounds are obtained separately in the form of different fractions. Further purification may be effected for example by crystallization, distillation or chromatography.
It is unexpected that the anodic oxidation of compounds I to II in the presence of a cathodic production of a multiplicity of organic compounds in an undivided cell is accomplished in good yields because compounds I, acetals and orthoesters, are themselves reactive compounds.